Re: [PATCH 2/3] hotplug-memory.c: enhance dlpar_memory_remove* LMB checks

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, May 12, 2021 at 05:35:39PM -0300, Daniel Henrique Barboza wrote:
> 
> On 5/3/21 10:02 PM, David Gibson wrote:
> > On Fri, Apr 30, 2021 at 09:09:16AM -0300, Daniel Henrique Barboza wrote:
> > > dlpar_memory_remove_by_ic() validates the amount of LMBs to be removed
> > > by checking !DRCONF_MEM_RESERVED, and in the following loop before
> > > dlpar_remove_lmb() a check for DRCONF_MEM_ASSIGNED is made before
> > > removing it. This means that a LMB that is both !DRCONF_MEM_RESERVED and
> > > !DRCONF_MEM_ASSIGNED will be counted as valid, but then not being
> > > removed.  The function will end up not removing all 'lmbs_to_remove'
> > > LMBs while also not reporting any errors.
> > > 
> > > Comparing it to dlpar_memory_remove_by_count(), the validation is done
> > > via lmb_is_removable(), which checks for DRCONF_MEM_ASSIGNED and fadump
> > > constraints. No additional check is made afterwards, and
> > > DRCONF_MEM_RESERVED is never checked before dlpar_remove_lmb(). The
> > > function doesn't have the same 'check A for validation, then B for
> > > removal' issue as remove_by_ic(), but it's not checking if the LMB is
> > > reserved.
> > > 
> > > There is no reason for these functions to validate the same operation in
> > > two different manners.
> > 
> > Actually, I think there is: remove_by_ic() is handling a request to
> > remove a specific range of LMBs.  If any are reserved, they can't be
> > removed and so this needs to fail.  But if they are !ASSIGNED, that
> > essentially means they're *already* removed (or never added), so
> > "removing" them is, correctly, a no-op.
> > 
> > remove_by_count(), in contrast, is being asked to remove a fixed
> > number of LMBs from wherever they can be found, and for that it needs
> > to find LMBs that haven't already been removed.
> > 
> > Basically remove_by_ic() is an absolute request: "make this set of
> > LMBs be not-plugged", whereas remove_by_count() is a relative request
> > "make N less LMBs be plugged".
> > 
> > 
> > So I think remove_by_ic()s existing handling is correct.  I'm less
> > sure if remove_by_count() ignoring RESERVED is correct - I couldn't
> > quickly find under what circumstances RESERVED gets set.
> 
> RESERVED is never set by the kernel. It is written in the DT by the
> firmware/hypervisor and the kernel just checks its value. QEMU sets it in
> spapr_dt_dynamic_memory() with the following comment:
> 
> 
>             /*
>              * LMB information for RMA, boot time RAM and gap b/n RAM and
>              * device memory region -- all these are marked as reserved
>              * and as having no valid DRC.
>              */
>             dynamic_memory[0] = cpu_to_be32(addr >> 32);
>             dynamic_memory[1] = cpu_to_be32(addr & 0xffffffff);
>             dynamic_memory[2] = cpu_to_be32(0);
>             dynamic_memory[3] = cpu_to_be32(0); /* reserved */
>             dynamic_memory[4] = cpu_to_be32(-1);
>             dynamic_memory[5] = cpu_to_be32(SPAPR_LMB_FLAGS_RESERVED |
>                                             SPAPR_LMB_FLAGS_DRC_INVALID);
> 
> 
> The flag is formally described in LOPAR section 4.2.8, "Reserved Memory":
> 
> "Memory nodes marked with the special value of the “status” property of
> “reserved” is not to be used or altered by the base OS."
> 
> 
> This makes me confident that we should check DRCONF_MEM_RESERVED in
> remove_by_count() as well, since phyp needs do adhere to these semantics and
> shouldn't be able to remove a LMB marked as RESERVED.

Right.  I doubt it would have caused a problem in practice, because
I'm pretty sure we should never get an LMB which is RESERVED &&
ASSIGNED, but it's probably safer to make it explicit.


-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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